Most types of modern electronic components (e.g., metal oxide semiconductor field effect transistors--MOSFETs) are susceptible to damage due to static electrical discharge. The gate-source breakdown in MOSFET occurs at about 100 volts, but due to the very high input impedance between gate and source (about one T-ohm), even a low-energy source such as static discharge can cause the MOSFET breakdown voltage to be exceeded.
When a MOSFET suffers breakdown, its gate oxide layer is permanently shorted, rendering the device useless. Gate protection devices are therefore incorporated in almost all MOSFET integrated circuits to prevent gate-oxide breakdown. Such protective circuits significantly reduce the input impedance of the gate prior to actual gate breakdown, making the device more resistant to destruction, especially once the device is installed in a circuit. Nevertheless, precautions must be taken. when installing and/or testing MOSFET devices to ensure against dangerous static electric discharge.
One common technique for avoiding static electric discharge is to electricalIy connect all objects which may come into close proximity to MOSFET devices to ground potential through resistances which are significantly less than the MOSFET input resistance. Since MOSFET devices may themselves hold a static electric charge, it is undesirable to connect such objects directly to ground potential--since this would actually encourage static electrical discharges. Instead, all persons, tools, and work surfaces which may come into proximity with the sensitive MOSFET devices are typically connected to ground potential through leads having a resistance somewhere between 10.sup.6 and 10.sup.10 ohms. Such relatively large resistances are high enough to prevent static electric "sparks" from being generated, and yet are low enough compared with the input impedance of a MOSFET to gradually and safely discharge any built-up static electric charge to ground potential.
Therefore, a person should attach a "wrist strap" around his wrist before handling MOSFET devices, and connect the wrist strap to ground potential through a high-resistance lead. Any static electric charge which might tend to build up on the person is slowly discharged to ground through the wrist strap and lead (rather than rapidly discharging via an air gap to another surface)--and likewise, static charges which have built up on other bodies are prevented from discharging rapidly to the person because of the relatively high resistance between the person and ground.
Similarly, all work with MOSFET devices is desirably performed on a conventional static dissipative work surface electrically connected to ground potential. Finally, all electrically conductive tools (such as needle-nose pliers, and the like) are electrically connected (through high-resistance leads) to ground potential, all in an effort to minimize static electric discharges between MOSFET devices and any other objects.
It would be desirable to continuously monitor the resistance of, for example, a dissipative work mat to ground potential so that faulty (or forgotten) ground connections can't cause destruction of expensive electrical components.
Simple testers which determine whether a body is shorted to ground potential and/or has an "infinite" resistance to ground are known. Unfortunately, resistances to ground which are acceptable for preventing rapid static discharges fall within a relatively large range between about 10.sup.6 ohms and 10.sup.10 ohms. A dissipative work surface having a resistance to ground which is less than on the order of 1 megohm can draw static electrical discharges from charged MOSFET devices. If such objects have a resistance to ground which is greater than about 10.sup.10 ohms, the objects may themselves hold static electric charges and cause spark discharges to MOSFET devices brought into proximity with them. Any resistance to ground which falls into the range between these two limits is acceptable.
Of course, resistance measuring devices with adjustable ranges are, in general, known. However, there is a great need for a relatively simple circuit which can automatically, continuously check the resistance between an object and ground potential, and can warn an operator whenever that resistance falls outside (either below or above) a predetermined resistance range--that range corresponding to the wide range of resistances acceptable for handling sensitive (e.g. MOSFET) devices.